Failed nuclear reactor fuel detection apparatus

ABSTRACT

Apparatus for detecting failed or leaking fuel rods in fuel assemblies in a nuclear reactor for power generation are described. A gripper for the fuel assembly is combined with a sampling tube for sucking up a coolant sample after the temperature of the fuel rods has been increased to a level causing increased emission of radioactive residues from a leaking rod. For the BWR, the gripper couples to the fuel assembly channel while in the reactor core in such manner as to isolate in a substantially air-tight manner the fuel assembly. Then an air bubble is introduced depressing the coolant level causing the desired temperature increase. For the PWR, the gripper lifts the fuel assembly into an overhead container to isolate same, and then heat is supplied via axuiliary heaters to raise the temperature of the fuel rods. To increase the sensitivity, the coolant sample is taken from below the upper fuel rod support.

Uite States atet 11 1 1 3,801,441

Jones Apr. 2, 1974 FAILED NUCLEAR REACTOR FUEL DETECTION APPARATUS 57ABSTRACT [75] Inventor? Cecil Jones, Hamden Colm- Apparatus fordetecting failed or leaking fuel rods in [73] Assignee: Transfer SystemsIncorporated, Ne fuel assemblies in a nuclear reactor for power genera-Haven Conn men are descrlbed. A gripper for the fuel assembly iscombined with a sampling tube for sucking up a cool- Flledl 12, 1971 antsample after the temperature of the fuel rods has [21] Appl No; 188,151been increased to a level causing increased emission of radioactiveresidues from a leaking rod. For the BWR, the gripper couples to thefuel assembly chan- U.S. nel in reactor core in uch manner as to isolatein a ubstantially manner the fuel assem- [58] Field of Search 176/19 R,19 LD Then an bubble is introduced depressing the coolant level causingthe desired temperature in- References Cited crease. For the PWR, thegripper lifts the fuel assem- UNITED STATES PATENTS bly into an overheadcontainer to isolate same, and 3.219536 11/1965 Butler et al 176/19 LDheat is pp via axuiliafy heaters raise the 3,234,101 2/1966 B nh d 17 191 temperature of the fuel rods. To increase the sensitiv- 3,393,l257/1968 Jackson... 176/19 R ity, the coolant sample is taken from belowthe upper 3,575,802 4/1971 Gelius 176/19 R fuel rod upport,

Primary Examiner-Reuben Epstein 5 Claims 7 Drawing Figures TO HOIST TO[:1 /9 m s F '1 43 56 i i {I} I 43 use PMEMEWR 219M 3.801.441

SHEH 3 OF 4 INVENTOR. CECIL R. JONES ATTORNEY PATENIEDAPR 2 19743,801,441

saw u or 4 INVENTOR. CECIL R. JONES Fig.6 Y l TTORNEY FAILED NUCLEARREACTOR FUEL DETECTION APPARATUS This invention relates to apparatus andmethod for detecting failed fuel rods in fuel assemblies used in powergenerating nuclear reactors.

BACKGROUND OF INVENTION There are several methods employed in the artfor detecting failed fuel elements. The method commonly employed in apressurized water reactor (PWR) is to lift out of the reactor core thefuel assembly to be tested, move it out of the reactor well into anadjacent pool, place it into a container in the adjacent pool,

' close the container, and after waiting thirty minutes or more, take awater sample from the container and test for the presence of certainradioactive isotopes emitted by leaking fuel rods. This method is verytime consuming and cumbersome. The total time required for the entireprocess can be several hours for each fuel assembly.

The method commonly employed for a boiling water reactor (BWR) involveslocating an apparatus over the fuel assembly while in the reactor core,and then causing the apparatus to seal to the upper core grid plate by ametal to metal contact, after which an air bubble is introduced todepress the coolant level causing increased heating of the fuel rods,and then a water sample is taken at a location just above the fuel rods.The problems involved here are that the pressure caused by the airbubble may cause lifting of the apparatus breaking the seal to the grid.Another disadvantage is that the water sample taken from above the fuelrods may not contain a sufficiently high concentration of the isotoperesidues affording a satisfactory indication of the presence of aleaking fuel rod. Also, certain short-lived isotopes especiallycharacteristic of leaking fuel rods may no longer be present because ofradioactive decay by the time they diffuse from the leak to above thefuel rods where the sample is taken.

In my copending application, Ser. No. 178,175, filed Sept. 7, 1971, Idescribe and illustrate a novel gripper apparatus for gripping andlifting a fuel assembly for a BWR or PWR. My novel gripper comprises agripper tube supported by an overhead hoist and terminating at itsbottom in a square opening with beveled edges. The beveled edges areadapted to mate with corresponding beveled edges at the top of thechannel ofa novel BWR fuel assembly also described in said copendingapplication Ser. No. 178,175. Mounted on the inside of the gripper tubeon horizontal square guide pins for lateral sliding movement relativethereto are a plurality of depending gripper fingers terminating inlateral projections for engaging recesses in the fuel assembly channelor housing for lifting same. The gripper fingers are moved outwardlyinto engagement with, or inwardly out of engagement with, the fuelassembly by a rotatable actuator mechanism located within the grippertube and comprising a central square tube having at its bottomhorizontal cam segments containing adjacent its peripheral edgeupstanding cam pins adapted to engage an overlying cam follower securedto the gripper fingers. Means comprising an air cylinder driven actuatortube are provided to rotate the square tube causing rotation of the campins causing the follower and attached fingers to be moved laterallyalong their supporting guide pins outwardly or inwardly depending uponthe direction of rotation. My novel gripper also provides a springbiased lock to prevent disengagement of the gripper fingers while a loadis applied, and also permits complete disassembly of the apparatus fromabove in case of malfunction.

In my copending application, Ser. No. 156,446, filed June 24, 1971, Idescribe and illustrate apparatus for testing conventional BWR fuelassemblies for leaking fuel rods. The apparatus comprises a structureadapted to be brought down over the reactor core into gasket sealingrelation with the upper core plate over one or more fuel assemblies toisolate them in a substantially air tight manner from the remaining fuelassemblies. A conventional internal grapple then is secured to the fuelassembly handle. Means are provided to pump air into the isolatedassemblies forcing out the coolant water normally present until thewater level is depressed to the level of the core plate within theisolated fuel assemblies. Due to the reduced cooling, the isolated fuelassemblies soon overheat, causing any leaking fuel rods present to emitinto the water remaining a higher concentration of characteristicradioactive material. Samples of the water remaining are then taken viaa sampling tube, a process called sipping, and the samples analyzed forthe presence of certain isotopes characteristic of leaking fuel rods. Inthis sipping process, the water samples are taken at or below the levelof the core grid.

The sipping arrangement described in copending application Ser. No.156,446 may have certain disadvantages which limit its potential use.One possible disadvantage is that effecting a tight seal to the coregrid to isolate the fuel assemblies is sometimes difficult if the gridand sampling apparatus are not precisely oriented with respect to eachother. Another disadvantage is that the apparatus described is notsuitable for the novel fuel assembly described in my copendingapplication, Ser. No. 178,175 which lacks a handle.

The sipping process described above also is not applicable to a PWRbecause coolant circulates freely between all the fuel assemblies and isnot channelized as in the BWR fuel assembly.

The present invention has as its main object the provision of apparatusfor isolating directly the fuel assembly of a BWR or PWR and alsoproviding for obtainment ofa coolant sample well below the top of thefuel rod assembly or bundle to afford a more accurate test for thepresence of a leaking fuel rod.

SUMMARY OF THE INVENTION The invention takes different forms whenapplied to a BWR or PWR. For the BWR, the invention is based upon therecognition that my novel fuel assembly described in copendingapplication Ser. No. 178,175 lends itself quite readily to a sippingprocess using a modified gripper apparatus of the type depicted in saidcopending application Ser. No. 178,175. In this embodiment, thesubstantially air-tight seal to isolate the fuel assembly is effectedvia the mating bevelled edges of the fuel assembly channel top and thegripper tube bottom. The sampling tube is mounted in the modifiedgripper in a position to register with an unoccupied opening in theupper tie plate for the assembled fuel rods, and thus can be extendeddown below the upper tie plate in a position to obtain a coolant samplewith a higher concentration of radioactive residues, especiallyshort-lived isotopes, from a leaking fuel rod if present.

For application to the PWR, the invention provides a gripper locatedover the reactor core for lifting up a fuel assembly into an overlyingcontainer which can be substantially closed off after receiving the fuelassembly to isolate it and a given quantity of coolant. Means areprovided within the isolation tube to heat the enclosed fuel assemblycausing the emission of radioactive residues from a leaking fuel rodinto the isolated coolant, and the coolant sipped via a sampling tubeintroduced below the fuel assembly top nozzle to test for the presenceof such residues.

The invention will now be described in greater detail with reference tothe accompanying drawings wherein:

FIG. 1 is a partially cross-sectional, partially elevational andschematic view of one form of my testing apparatus for use with a BWR.The solid line construction is for use with a single fuel assembly. Thephantom view alongside shows how the single unit is readily multipliedfor use with multiple fuel assemblies;

FIG. 2 is a partly cross-sectional, partly elevational view of one formof my testing apparatus for use with a PWR;

FIGS. 3 and 4 are cross-sectional views, and FIG. 5 is a bottom view ofthe apparatus of FIG. 2 taken along the lines 3-3, 4-4, and 55respectively;

FIG. 6 is a partly cross-sectional, partly elevational view of thegripper employed in apparatus of FIG. 2;

FIG. 7 is a cross-sectional view of the gripper of FIG. 6 taken alongthe line 7-7.

BWR EMBODIMENT Reference is had to my copending application, Ser. No.178,175 for a detailed description of the novel gripper apparatus andBWR fuel assembly described and claimed therein. Repeating this detaileddescription is unnecessary, and thus the present specification willconcentrate on the parts of the gripper apparatus that have beenmodified to achieve the present objects.

FIG. 1 illustrates part of a reactor core 10 of a conventional BWR withan upper core grid plate 11 supporting in parallel position a pluralityof fuel assemblies 12 of the type depicted in my copending application,Ser. No. 178,175. These fuel assemblies differ from the prior artversions mainly in the use of identical fuel rods 13 and thus capable ofoccupying any position within the fuel assembly, and the provision of aspring 14 mounted, removable upper tie plate 15 for fixing the positionsof the parallel fuel rods. The spaced fuel rods, only some of which areshown, are held within the channel or housing 117 by the upper 15 andlower (not shown) tie plates and intermediate spacers 18, only one ofwhich is shown. As will be noted, the upper tie plate 15 is apertured 19to permit the free flow of coolant therethrough. Many of the aperturesare unoccupied, and my invention will provide for passage ofa samplingtube shown at 20 through one of the unoccupied apertures into the spacebetween the upper tie plate 15 and the adjacent spacer 18.

The top end of the channel or housing forms a square opening surroundedby walls 21 terminating in bevelled edges 22, which are employed as partof the sealing system to isolate the fuel assembly.

The modified gripper apparatus 25 is brought down via a coupling 9 by asuitable hoist not shown over the reactor well. It comprises, similarlyto what is described in my copending application, Ser. No. 178,175 agripper tube 26 terminating at its bottom in a square opening defined bybeveled edges 27 dimensioned to fit within and mate with the bevelededges 22 of the channel 21 forming a substantially water-tight andair-tight seal. The gripper tube 26 is tightly held to the channel byfour gripper fingers 28, terminating in lateral projections 29 engagingholes 31 in the channel wall 21. In my copending application, such acoupling is used to lift the fuel assembly when the gripper is lifted.In the present invention, the coupling is employed to prevent lifting ofthe gripper due to air pressure when air is pumped into the sealedcompartment 25 to reduce the water level.

The gripper fingers 28 are supported by square guide pins 32 mounted onthe gripper tube 26, and which allow the gripper fingers to movelaterally inwardly to release the fuel assembly, or outwardly to gripthe assembly. Such lateral movement is accomplished in the same manneras depicted in my copending application, Ser. No. 178,175 via a squaretube 33 secured at its bottom to horizontal cam segments 34 and cam pins30 equal in number to the number of gripper fingers. Each gripper fingeris provided with a suitable cam follower 35. Rotation of the camsegments 34 out of the drawing plane causes rotation of the cam pinspushing the cam follower 35 and attached finger 28 outwardly. Reversingthe rotation reverses the movement. The rotation is imparted by asuitable hydraulic cylinder 36 coupled to the upper end of the squaretube 33. (FIG. 7 depicts a comparable arrangement for the PWR grippervia an intervening activator tube.) The fixed remote end of the cylinderis coupled to the outer gripper tube 26.

My copending application, Ser. No. 178,175 also provides a spring biasedlocking system to prevent disengagement of the gripper fingers whilethey are supporting the fuel assembly load, but since the fuel assemblyis not lifted out of the core in the present invention, such a safetyfeature is unnecessary and has been omitted.

The gripper 26 is also provided with means for accommodating in asubstantially air-tight manner a movable sampling tube 20. The samplingtube 20 engages a sleeve 38 secured in a sampling guide tube 39depending from an upper support plate 41) from which the gripper tube isalso suspended. The sleeve 38 is provided with inset O-rings 41 enablingsubstantial sealing to the sampling tube awhile permitting verticalmovement of the sampling tube through the guide 39.

The sampling tube 20 itself is supported by an overhead plate 413aligned relative to the gripper upper plate 40 via the piston 7 of anair cylinder 6 providing for limited vertical movement of the plate 43relative to the gripper plate 40.

In operation, after shutting down of the reactor and opening of thereactor well exposing the core and fuel assemblies, the gripper tube 26is oriented over the fuel assembly to be tested and then slowly loweredinto position until the gripper flanges 27 seat within the channelflanges 22. During this initial phase, the sampling tube 20 supportplate 43 is in a raised position relative to the plate 40 preventingpossible contact of the sampling tube end 511 with the upper tie plate115. Then the gripper air cylinder 36 is activated rotating the squaretube 33 and causing the gripper fingers 28 to engage the channelrecesses 31 locking the gripper to the selected fuel assembly as shown.The sampling guide tube 39 is now aligned over an unoccupied hole 46 inthe upper tie plate 15.

Next, the sampling tube support structure 43 is slowly lowered by aircylinder 6 to the position shown in the drawing lowering the samplingtube 20 through the guide tube 39 until its open end 50 passes throughthe upper tie plate aperture 46 but before the next lowest spacer 18 isreached. Then a suitable air pump 51 connected to inlet 52 is actuatedpumping air into the water filled enclosed space which displaces thewater. The only exit available for the displaced water is the openbottom '53 of'the fuel assembly channel. Thus, as

air is continually pumped in, the water level within the gripper-channelcompartment falls. The water level is stabilized at just above thesampling tube opening, as shown by the dashed line 54. The reducedcooling effected by the reduced water level causes in a short time of,say, -30 minutes a substantial rise in the temperature of the fuel rods13 in the selected fuel assembly. As the temperature climbs to about 150F or higher, radioactive isotopes emitted into the remaining water by aleaking fuel rod intensifies, increasing considerably the concentrationtherein. After the aforementioned delay time, a sample of the waterstill left in the fuel assembly is sucked up through the sampling tube26 and carried by a flexible tube 56 into a suitable vial (not shown),and, as explained in my copending application, Ser. No. 156,446,subjected to a suitable analysis to determine the presence therein ofcertain isotopes characteristic of a leaking fuel rod. If the analysisindicates leakage is present, the fuel assembly is designated for laterreplacement.

As mentioned earlier, improved sensitivity is obtained because the watersample is taken from well below the upper tie plate of the fuelassembly, and thus will contain a higher concentration of especiallyshortlived isotopes such as Iodine 131, typically generated by leakingfuel rods, than in the case of a sample taken from the level of the topof the core grid.

After the sampling process is completed, the sampling tube is withdrawnby raising its support 43, the air cylinder 36 for actuating the grippermechanism reversed, and upon release of the gripper fingers, the gripperapparatus can be lifted by the hoist and moved over another fuelassembly to be tested. For removal of a defective fuel assembly, thegripper of my copending application, Ser. No. 178,175, is employed.

The gripper-sipper apparatus of my invention is adapted to engage andsip from a single BWR fuel assembly, as illustrated in solid lines inFIG. 1, or from multiple fuel assemblies, preferably four at a time ormultiples of four. This is obtained by multiplying the structuredepicted in FIG. 11 so as to encompass several adjacent fuel assemblies.This is illustrated in phantom in FIG. 1, which shows a second fuelassembly 16 being simultaneously gripped by a second gripper apparatus61 also provided with its own sampling tube 62. Where several grippersare provided, they can be united into a single mechanism. This is simplyaccomplished by joining the adjacent gripper tube walls 26 to form acommon wall 63 terminating in a V-shaped slot 641 for simultaneouslysealing to adjacent channel walls 21 of adjacent fuel assemblies.Similarly, the sampling tube supports 43 can be combined into a singlestructure movable simultaneously. An air vent 66 provided in the commongripper wall 63 serves to combine the separate spaces into a singlecombined isolated space comprising plural fuel assemblies, thus allowingthe use of a single air pump.

The aforedescribed BWR embodiment provides for isolation of individualor plural adjacent fuel assemblies while still retained within thereactor core. This is possible because each fuel assembly is isolated,insofar as the coolant is concerned, from its neighbor except by itschannel opening 53 at the bottom. Thus, the above embodiment is notsuitable for a PWR, where coolant circulates freely between the fuel rodclusters or fuel rod assemblies. To isolate the latter for sippingpurposes requires a modified system.

PWR EMBODIMENT In this embodiment of my invention, the fuel assembly isisolated by relocating it while still in the reactor well and underwater in an overhead isolation container.

This is depicted in FIG. 2. Part of the reactor core is shown at 70, andcomprises a plurality of adjacent fuel assemblies 72 of conventionalconstruction comprising a plurality of parallel spaced fuel rods 73 heldin position at their upper ends by an upper grid assembly 74 and belowby intermediate grid assembly 75. Located over the assembled fuel rodsis a top nozzle 76 having an apertured grid bottom 77 for aligningcontrol rods of a control rod assembly 78. A triangular element ispositioned on top.

As described in my copending application Ser. No. 178,175, such a fuelassembly can be gripped and lifted by the same gripper apparatusdescribed therein for the BWR slightly modified insofar as the ends ofthe gripper fingers are concerned to enable them to be extended insideand under the edges of the top nozzle 76. That same gripper of mycopending application is employed herein to grip and lift one of thefuel assemblies. The only change made is to modify slightly the internalsquare actuating tube to accommodate a guide for the sampling tube.

FIG. 6 illustrates such a gripper apparatus including the actuating camsystem and safety lock used in a lifting operation. The same referencenumerals are used as in FIG. 1 for similar parts. It comprises a grippertube 26, gripper fingers 28 with modified extensions laterally slidableon guide pins 32. An inner square tube 121 contains the lateral camsegments 34 with cam pins 30 activating cam followers 35 secured to thefingers 28. The lock mechanism used in a lifting operation includes alocking finger 123 for engaging a rim 124 on the gripper tube andextending through a hole in a guide member 125 for an actuator tube 126which supports the square tube 121 by an annular compression spring 127.In this embodiment, as shown in FIG. 7, the actuating air cylinder 36 isconnected between the actuator tube 126 and the gripper tube 26.Rotation of the actuator tube 126 causes corresponding rotation of thesquare tube 121. The gripper structure mainly differs from the gripperof my copending application only in that a comer of the inwardlyextending horizontal shelf 130 of the square tube 121 bottom, which isdesigned to receive the top of the control rod assembly, has been cutaway to accommodate passage of the sampling tube 131. The sampling tube131 is aligned relative to the final assembly by means of an elongatedhollow square tube 132 which extends downward from and is supported bythe upper support plate 40 of the gripper and whose bottom is bevelled133 to seat within the top 134 of the square tube 121. A bore 135extends vertically through a solid corner 136 of the tube guide 132.Through the bore extends the sampling tube 131. The bore 135 is orientedover an unoccupied opening in the bottom grid 77 of the top nozzle 76,and the sampling tube 131 can thus extend down through the gripper,through the grid 77 at the bottom of the top nozzle and terminate in thespace below but above the upper grid assembly 75.

The gripper (FIG. 2) is located for vertical movement within anisolation container 90 comprising spaced concentric tubes 91, 92. Theinner tube 91 directly surrounds the gripper and its hoist and canaccommodate a lifted fuel assembly. The outer tube 92 is spaced from theinner tube. Both tubes extend from the reactor well surface, above thewater level W, all the way down almost to the core surface. The systemis supported by an overhead hoist (not shown). The outer tube 92 isconnected to or fixed to the hoist, and the inner tube 91 is verticallymovable relative to the outer tube. Vertical movement is obtained by asuitable air cylinder 93 mounted between the concentric tubes, with thecylinder 94 connected to the inner tube 91 and the piston 95 connectedto the outer tube 92. Suitable guides 96 are mounted on the outer tubeto guide movement of the inner tube.

Also located between the tubes at the lower side are vertical heatingelements 98. Four rod-like heating elements 98 are shown. The tubebottom is fitted with a trap door 100 which can be selectively openedand closed to seal off in a substantially water-tight manner the bottomof the inner tube 91. Any form of suitable remotely activated trap doorcan be employed. A preferred form employs actuation by movement of theinner tube 91. In the embodiment illustrated, the trap door comprisesfour circle quadrants or 90 sectors 101 each terminating in an upwardextending conical section 102 each of which is hinged 1113 at its upperedge to the bottom edge of the outer tube 92. At the junction of eachcircle sector 101 and conical section 102 is hinged a rod 105, whoseupper end is hinged 106 as shown to the inner tube 91, above its bottom.FIG. 2 shows all the trap door segments in their up position forming asubstantially water-tight seal along their abutting edges. When theinner tube 91 is moved downward, the door segments 101, 102 are flippeddownward completely opening the inner tube bottom. In the closedposition, the door segments also abut with their inner surfaces thebottom edge of the inner tube 91, which may be provided with a softgasket to assist in sealing to the trap door 100.

In operation, the tube assembly 90 is positioned over the fuel assemblyin the reactor core to be tested. The gripper apparatus is loweredthrough the inner tube past the open trap door 100 into contact with theselected fuel assembly 72 in the core, the actuator tube of the gripperoperated to grapple to the fuel assemble 72, and then the gripper andhooked fuel assembly lifted up out of the core 70 and into the innertube 91. When the entire fuel assembly has passed through the open trapdoor 100, the gripper hoist is stopped, and then the air cylinder 93activated to raise the inner tube 91, closing the trap door 100, andisolating the lifted fuel assembly and the enclosed coolant column 108within the inner tube. In order to raise the fuel assembly temperature,the auxiliary heaters 98 are activated and after a short time thedesired high temperature is achieved whereby the emission of isotopescharacteristic ofa leaking fuel rod is intensified, which isotoperesidues are emitted into the surrounding water column. Then, or beforethe heating step if desired, the sampling tube 131 is lowered throughthe bore 135 in its guide into a position below the top nozzle grid 77,and a suitable water sample sucked up and analyzed as described above.After testing, the heaters are deactivated, the inner tube 91 moved downto open the trap door 100, and the gripper can be lowered to replace thefuel assembly in its identical position formerly occupied within the"reactor core. 7 W

In this embodiment, the water level is not reduced by introducing an airbubble as was done with the BWR, but instead the required heating isobtained by the provision of auxiliary heaters. The same improvement intesting sensitivity is obtained by the taking of a water sample from thefuel assembly interior, below the top nozzle.

While the sipping method described for the PWR employing a tube throughthe gripper is preferred, since the confined water circulates freelythrough the fuel assembly housing other ways of taking a water samplemay also be used. For example, a sipping tube can be extended down alongthe wall of the inner tube 91 to the level of the fuel elements 73. Alsoseveral water samples can be taken at different locations within theisolation container 90 to further increase the sensitivity of the test.

While the principles of the invention have now been made clear inillustrative embodiments, it will be obvious to those skilled in the artthat many modifications in structure, arrangement, proportions,elements, ma terials, and components used in the practice of theinvention, which are particularly adapted for specific environments andoperating requirements, may readily be made therein without departingfrom those principles. The appended claims are therefore intended tocover and embrace any such modifications, within the limits only of thetrue spirit of the invention.

What is claimed is:

1. Apparatus for testing at least one fuel assembly of a BWR whilesurrounded by coolant in the reactor core for the presence of leakingfuel rods, said fuel assembly comprising a channel having an upstandingperipheral edge portion and an upper tie plate for supporting the upperends of an assembly of fuel rods and also containing openings free ofthe fuel rods; comprising gripper means comprising an outer member whichis substantially air-tight and having a depending lower edge portiondimensioned to mate with and substantially seal to the upstandingperipheral edge portion of the channel forming with the latter acontainer which is substantially air-tight, means coupled to the outermember for selectively locking to the fuel assembly channel, means forintroducing an air bubble into the container for depressing the coolantlevel causing the temperature of the fuel'rods to rise, and means forintroducing a sampling tube through the outer member and through the tieplate opening for removing a sample of coolant disposed beneath theupper tie plate.

2. Apparatus as set forth in claim 1 wherein the channel peripheral edgeportion comprises a bevelled edge,

extending through the gripper from the outer member wall to the vicinityof the channel and provided with means for sealing to the sampling tubewhen the latter is passed therethrough.

5. Apparatus as set forth in claim 1 and further comprising means forlowering and raising the gripper relative to the fuel assembly, asampling tube support, and means for raising and lowering the samplingtube support relative to the gripper.

1. Apparatus for testing at least one fuel assembly of a BWR whilesurrounded by coolant in the reactor core for the presence of leakingfuel rods, said fuel assembly comprising a channel having an upstandingperipheral edge portion and an uppEr tie plate for supporting the upperends of an assembly of fuel rods and also containing openings free ofthe fuel rods; comprising gripper means comprising an outer member whichis substantially air-tight and having a depending lower edge portiondimensioned to mate with and substantially seal to the upstandingperipheral edge portion of the channel forming with the latter acontainer which is substantially air-tight, means coupled to the outermember for selectively locking to the fuel assembly channel, means forintroducing an air bubble into the container for depressing the coolantlevel causing the temperature of the fuel rods to rise, and means forintroducing a sampling tube through the outer member and through the tieplate opening for removing a sample of coolant disposed beneath theupper tie plate.
 2. Apparatus as set forth in claim 1 wherein thechannel peripheral edge portion comprises a bevelled edge, and thegripper outer member lower edge portion also comprises a bevelled edge.3. Apparatus as set forth in claim 2 wherein the outer member comprisesdepending fingers having lateral extensions for engaging recesses in thechannel walls, means supporting the fingers for lateral movementrelative to the outer member, and means for actuating the fingersselectively to engage and disengage the recesses.
 4. Apparatus as setforth in claim 1 wherein the sampling tube introducing means comprises aguide tube extending through the gripper from the outer member wall tothe vicinity of the channel and provided with means for sealing to thesampling tube when the latter is passed therethrough.
 5. Apparatus asset forth in claim 1 and further comprising means for lowering andraising the gripper relative to the fuel assembly, a sampling tubesupport, and means for raising and lowering the sampling tube supportrelative to the gripper.